Non-crystalline Inorganic Constituents of Soil
- 6.7k Downloads
Non-crystalline inorganic constituents of soil, such as volcanic glasses, phytoliths, laminar opaline silica, allophane, and imogolite are introduced using optical and electron microscope images and energy dispersive X-ray (EDX) analysis. The Al-humus complex and Al-rich Sclerotia grains are also introduced. The volcanic glasses are formed from magma and can be categorized as primary. All of these non-crystalline inorganic constituents are found in volcanic ash soils. Among these, phytoliths can be found under vegetation in many other soils than volcanic ash soils. Formation of allophanic materials from fresh pumice is shown stepwise using polished sections to demonstrate microscopic distribution of elements and inorganic constituents. Allophane and imogolite are rich in Al whereas their parent material, volcanic ash, is silica-rich. Changes in morphological property and element concentration of volcanic ash or volcanic glass during the formation of these secondary non-crystalline constituents are discussed.
KeywordsPhytolith Allophanic Materials Opaline Silica Imogolite Volcanic Ash
Non-crystalline silicate and silica constituents in soil
Laminar opaline silica
The Al-humus complex is also covered in this chapter despite its partially organic structure because it is one of the major constituents of active Al. Active Al is important to the formation of Andisols and Spodosols, classification of these soils, and the phosphate sorption reaction.
Other non-crystalline materials which may be present in soils include non-crystalline phases of iron sulfide, iron phosphate, and aluminum phosphate. Non-crystalline iron sulfide and iron phosphate were covered in Chap. 5, and non-crystalline aluminum phosphate were covered in Sect. 6.4. The term “volcanic ash soils” encompasses all soils derived from volcanic ash, whereas Andisols are soils defined by Soil Taxonomy of United States Department of Agriculutre (USDA).
4.2 Volcanic Glass
Volcanic ash, in this monograph, refers collectively to volcanic ejecta or tephra, including pyroclastic fall and flow materials such as volcanic ash, cinders, lapilli, scoria, and pumice (Dahlgren et al. 1993). Volcanic glass is a non-crystalline silica-alumina material and is a major constituent of the volcanic ash ejected from volcanoes (Yamada and Shoji 1975; Heiken and Wohletz 1985). The morphological types of volcanic glasses include sponge-like, bubble-wall type (curved platy), fibrous, and berry-like (Shoji et al. 1993). The diameter of sponge-like and berry-like volcanic glasses can exceed 2 mm. The diameter of volcanic glass ranges below 2 μm. The color of volcanic glasses is related to the rock type of volcanic ash. Sponge-like, bubble-wall, or fibrous glasses are non-colored. Coloured volcanic glass is mostly berry-like with crystallites.
4.2.1 Chemical Composition of Volcanic Glasses
Shoji et al. (1975) classified volcanic ashes into five rock types based on the total SiO2 content. The five rock types are rhyolite, dacite, andesite, basaltic andesite, and basalt. Their total SiO2 content ranges are 100–70, 70–62, 62–58, 58–53.5, and 53.5–45%, respectively. Reclassification of 26 tephra reported by Shoji et al. (1975) using the updated classification of Le Maitre (2002), known as the total alkali silica classification (TAS diagram), yielded the same results for 22 tephra. Two of the remaining tephra were close to the boundary between andesite and dacite, and the other two lay between dacite and rhyolite, because the Na2O + K2O contents of all 26 tephra were below the boundaries between the four rock types (basalt, basaltic andesite, andesite, and dacite) and the corresponding trachy-types of the TAS diagram.
Volcanic ash is sorted by its particle size and the specific gravity of its constituent mineral particles during transportation in air. In the case of Tarmae-a (Ta-a) tephra (AD1739), Hokkaido, Japan, the heavy mineral content decreased with distance from source volcano. As the heavy mineral content decreased, the Fe and Mg content of Ta-a tephra also decreased while the Si content increased correspondingly (Mizuno et al. 2008).
The color of volcanic glasses is related to the rock type of the tephra. Volcanic ash of rhyolite, dacite, and andesite rock types is dominated by non-colored volcanic glasses including a slightly purplish one, whereas basalt and basaltic andesite rock type is dominated by colored volcanic glasses (Shoji 1986). The chemical composition of volcanic ash is also related to rock types. The Al2O3, FeO, MgO, and CaO content of 26 volcanic ash samples significantly, and Na2O, K2O and TiO2 also correlated with SiO2 content (Shoji et al. 1975). In nine non-colored volcanic glass samples, SiO2 content ranged between 74.24 and 77.89%, and Al2O3 content ranged between 12.65 and 14.71%. The variation in the concentrations of SiO2 and Al2O3 of the non-colored volcanic glasses are minor compared to that for the total SiO2 and Al2O3 concentrations of the volcanic ashes. In contrast, in glasses present in basaltic andesite and basalt volcanic ashes, the Al2O3, Fe2O3, MgO, and CaO contents increase with decreasing SiO2 content.
4.2.2 Sponge-Like Volcanic Glass
4.2.3 Bubble-Wall Type Volcanic Glass
4.2.4 Fibrous Volcanic Glass
4.2.5 Berry-Like Volcanic Glass
Figure 4.6 shows an example of a scoria particle or colored volcanic glass. The particle is dark and has many concavities (Fig. 4.6a, b). The EDX spectrum (Fig. 4.6c) of the dashed area (Fig. 4.6b) shows that Mg, Al, Ca, and Fe concentrations are high, and that the Si concentration is low compared with non-colored volcanic glasses (Figs. 4.2c, 4.3e, and 4.4c). Many plagioclase crystallites are visible in the magnified SEM image (Fig. 4.6d).
Brownish red particles can be found in Fig. 4.5c, and they are also scoriaceous. Figure 4.7a is a red scoriaceous particle and Fig. 4.7b is the SEM image of Fig. 4.7a that resembles Fig. 4.6b in that it has round concavities. The EDX spectrum (Fig. 4.7c) of the dashed square in Fig. 4.7b is similar to that of the dark-colored scoria particle (Fig. 4.6c) in terms of the high Mg, Al, Ca, and Fe concentration compared with those for Figs. 4.2c, 4.3e, and 4.4c. In the magnified SEM image (Fig. 4.7d), small plagioclases are identified by EDX analyses. The brownish red color of the particle shown in Fig. 4.7a can be explained by high-temperature oxidation of iron.
4.3 Secondary Non-crystalline Inorganic Constituents
Non-crystalline inorganic constituents in this section are newly formed in soil or plants whereas volcanic glasses are of magmatic origin.
4.3.1 Allophane and Imogolite
The outside of allophane and imogolite resembles a gibbsite sheet (Fig. 4.8c). The dehydration of three H2O molecules from the three front OH groups and the three OH groups of Si(OH)4 yields the aluminosilicate wall of allophane (Fig. 4.8c). Because there is no Si-O-Si bond, allophane and imogolite are nesosilicates. The Si-OH groups point towards the inside of the spherules and tubes, as shown in Fig. 4.8a, b, respectively. Allophane having a Si:Al ratio of 1:2 is called Al-rich allophane and is the major type found in Andisols. When the atomic Si:Al ratio of allophane approaches 1, it is considered Si-rich allophane, the Si dimer is thought to increase. The Si-rich allophane is sometimes found in the weathered pumice layer of the deeper part of Andisol profiles.
As shown in Fig. 4.9a and c, the imogolite gel film exists at the voids among pumice particles, and it contains not only imogolite but also allophane, suggesting that these films are formed through precipitation reactions. Allophanic materials form as altermorphs of vesicle walls inside the pumice particle as shown later in Figs. 4.26 and 4.29. Thus, two types of occurrences are possible for allophane in the weathered pumice .
The allophane and imogolite content can be estimated by oxalate extraction as 7.1 times Sio, where Sio denotes the amount of oxalate-extractable Si (Parfitt and Henmi 1982; Parfitt and Wilson 1985). The occurrence of allophane and imogolite is further discussed in Sect. 4.4.
4.3.2 Laminar Opaline Silica
Figure 4.10a shows a profile of Udivitrand in Hokkaido, Japan. In the clay fraction of the uppermost A horizon, laminar opaline silica particles can be easily found. The EDX spectrum of laminar opaline silica is highly dominated by Si (Fig. 4.10c).
Phytoliths, or plant opals, are a type of biogenic opals (Wilding et al. 1977; Drees et al. 1989). Phytoliths are typically found in the particle size fraction of 5–50 μm and are larger than laminar opaline silica (Fig. 4.10b). Phytoliths are abundant in A or buried A horizons. Phytoliths are non-crystalline silica, formed in plant cells. As the morphological form of a phytolith is specific to plant species to some extent, the form of phytoliths in a buried A horizon can be used to estimate the paleovegetation of the buried A horizon (Kondo et al. 1988). Phytoliths possibly benefit plants by contributing to Si nutrient cycling.
Biogenic opals, laminar opaline silica , and volcanic glasses are non-crystalline. Morphological characteristics are used to identify non-crystalline inorganic constituents. Although XRD is a powerful tool for the identification of crystalline constituents, it is not effective for non-crystalline inorganic constituents. Elemental composition is an useful tool for identification of non-crystalline inorganic constituents. EDX is effective for the determination of the approximate elemental composition of particles in soil.
In order to examine the andic soil properties in USDA Soil Taxonomy and andic properties in World Reference Base for Soil Resources, glass counting is needed for the young soils (Eden 1992). In glass counting, the 2–0.02mm fraction is used and a distinction between volcanic glass and phytoliths is necessary. Subdivision of the 2–0.02mm fraction into 2–0.2, 0.2–0.05 and 0.05–0.02mm fractions is effective in allowing each particle to be identified. Fractionation of these particle-size fractions should be done after H2O2 digestion, dithionite-citrate-bicarbonate treatment, and wet sieving. If data on the smaller particle-size fraction are needed, the 0.02–0.005-mm fraction can be prepared, removing the fraction less than 0.005-mm fraction by a dispersion–sedimentation and siphoning procedure. For quantitative purpose, eight repetitions of this procedure must be performed.
A few characteristics of the 2–0.005mm particle-size fraction can be noted. Phytoliths are not included in the 2–0.2mm fraction, and are not prevalent in the B and C horizon soils. Volcanic glasses are present in all size fractions, although the abundance of volcanic glass in each particle-size fraction changes depending on the individual sample (Yamada and Shoji 1975).
Only the wholly visible particles in Fig. 4.12 (numbered particles 1 through 15) were targeted for interpretation. Figure 4.12b, c are element maps of Al and Si, respectively. Particles 7, 13, and 14 are seen to contain Al. All the particles contain Si. Particles 1 to 6, 8 to 12, and 15 contain few elements other than Si, suggesting that they are silicas. Among these silica particles, a micrograph with crossed polarizers (Lynn et al. 2008) (Fig. 4.12d) suggests that particles 3, 5, 6, 8, 9, 14, and 15 are anisotropic, which indicates that they are crystalline silica minerals. The same results were obtained after turning the rotating stage of the polarizing microscope. Other silica particles (1, 2, 4, 10, 11, and 12) are isotropic, suggesting that they are phytoliths. Particle 13 is identified as volcanic glass from its vesicular morphological properties in addition to its elemental composition (Fig. 4.12b, c) containing both Si and Al. Particles 7 and 14 appear to be aluminosilicates. Although particle 7 is isotropic, its Al content is too high to allow for identification as a volcanic glass.
Diatoms, which are also biogenic opals, are also found in the clay or silt fractions of A horizon soils. Diatoms can be identified by their characteristic frustule. As laminar opaline silica is almost always smaller than 5 μm, it does not affect the glass counting of the 2–0.02 mm fraction.
Thus, by using both polarizing microscope and SEM -EDX, the ability to identify non-crystalline soil constituents such as volcanic glass and phytoliths is greatly enhanced. Remaining issues include a few phytoliths and diatoms that are weakly anisotropic, and crystalline particles completely covered by volcanic glass. Although some iron minerals are not transparent under optical microscope, they can be identified using an Fe element map.
4.3.4 Al-Humus Complex
Al-humus complex (Al-humus) is one of the active Al materials in Andisols and Spodosols. Al-humus was recently reviewed by Takahashi and Dahlgren (2016). This section is focused on Al-humus in Andisols, because in these soils Al-humus is much less mobile, whereas that in Spodosols appears more mobile and is related to eluviation–illuviation processes (Ito et al. 1991). The justification for covering Al-humus despite its partially organic structure is that (i) it is highly reactive with phosphate, (ii) Al-humus formation appears to be competitive with allophane and imogolite in Andisols depending on the pH and organic matter content, and (iii) it is the major active Al form in the A horizons of non-allophanic Andisols.
4.4 Andisols: Soils Dominated by Non-crystalline Inorganic Constituents
The Si/Al atomic ratio of major constituents in volcanic ash soils
Constituents in volcanic ash soils
Si/Al atomic ratio
Non-colored volcanic glass
Colored volcanic glass
Soil formation products
The distribution of Andisols is largely determined by the locations of volcanic regions. The major volcanic regions in the world are the Circum-Pacific volcanic zone, the Hawaiian Islands, the Sumatra-Java-Lesser Sunda Islands, Eastern Africa, the Mediterranean area, Iceland, the Azores, and the Canary Islands (Sievert et al. 2010; Machida 2002a). In the case of huge volcanic eruptions, which occur intermittently, interspersed with dormant periods of hundreds or thousands of years, a large amount of volcanic ash affects wide areas (Machida 2002a). Volcanic ash deposits in these areas are important parent materials of soils.
Figure 4.14 shows a schematic of Andisol formation from volcanic ash. Highly explosive eruptions are caused by viscous, Si-rich magma, which form non-colored volcanic glass . In the mid-latitudes, volcanic ash is mainly distributed on the eastern side of volcanoes owing to the influence of the strong westerly winds, whereas at low latitudes the ash is found either all around the volcano or slightly to the western side owing to the influence of the weak trade winds.
Volcanic ash deposits on the slopes of volcanoes are further transported by water as volcanic mudflows (lahar). In the case of the Mt. Pinatubo ash deposit in 1991, a large amount of volcanic ash was transported to lower elevations along large rivers during the rainy season ( Fig. 2.7). Mudflows continued to occur for several years after the eruption.
The occurrence of intermittent huge eruptions accompanied by large ash falls is evident from the thaptic property in the vicinity of a volcano. The thaptic property is the existence of a buried Andisol profile under a new volcanic ash soil profile, forming a multisequum profile (Fig. 4.14a, b). Within the volcanic ash-affected area, the amount of ash fall and the size of the ash particles decreases as the distance from the volcano increases. As a result of thin and cumulative deposition of volcanic ash, a thick humus-rich A horizon, which is a characteristic of the Pachic subgroup in USDA Soil Taxonomy, is formed (Fig. 4.14c). Andisols are typically formed on hills and uplands with good drainage. Leaching losses of Si from volcanic ash cause higher Si concentrations in the rivers in the vicinity of volcanoes than in those in non-volcanic areas. Although volcanic ash also falls in the lowlands, marshes, or basins with restricted drainage areas, the weathering of volcanic glass is slow and halloysite is formed, possibly because high Si concentrations are maintained in the soil water. The formation of smectite from volcanic glass is also reported under hydrothermal conditions in the laboratory (Tomita et al. 1993; Cuadros et al. 1999).
Although humus-rich horizons are generally dark-colored, highly black colored A horizons are often found beneath grass vegetation. The grass vegetation was maintained even under humid climates through burning by ancient people. Under forest vegetation, the color of A horizons is dark brown. The difference in color between these two types of A horizons is due to the differences in their humic acid types. The humic acid, under grass vegetation is A-type, which is highly humified and rich in aromatic groups whereas that under forest vegetation is either P-type or B-type. The A-type humic acid is separated from other types of humic acid by having a melanic index lower than 1.7, whereas the other types of humic acid have melanic indices of 1.7 or higher (Shoji et al. 1993).
Andisols are characterized by high active Al and Fe contents. The active Al materials are allophane, imogolite, and Al-humus, and the active Fe material is ferrihydrite. These materials are mainly formed from volcanic glass. The typical Si:Al atomic ratios of non-colored and colored volcanic glasses are 5.0 and 2.4, respectively. These values are significantly higher than those for soil formation products such as kaolin minerals (often halloysite), allophane, imogolite, and Al-humus, as shown in Table 4.2. Morphological changes of volcanic glasses accompanying these changes in elemental composition were examined using polished sections of new pumice particles, partially weathered pumice particles and soil clods from Udands.
4.4.1 Fresh Pumice Particle
Figure 4.15f, g are the element maps of Al and Si, respectively. The cyan color used to indicate Si displays its second highest intensity for the major and glassy part of the pumice particle. The highest color intensity of Si may indicate that some quartz was included in the pumice. The magenta color chosen for Al is strongest for feldspar particles, and the intensity of the magenta color for the volcanic glass is lower than the cyan color intensity as suggested from the EDX spectra shown in Fig. 4.15b, e.
4.4.2 Partially Weathered Pumice Particle
Figure 4.18a, b shows element maps for Si and Al, respectively. The intensity of the cyan color, which indicates Si concentration, is somewhat weak at the rim, although similar low intensities are found in parts of the interior. The intensity of the magenta color, which indicates Al concentration, is especially high at the rim, for example at the site indicated by the white arrow labeled (d) in the SEM image (Fig. 4.18c), where Si concentration appears low. In contrast, the Si concentration still appears to be quite high at the site indicated by the white arrow labeled (e) in Fig. 4.18c. These observations can be confirmed by the EDX spectra (lower right of Fig. 4.18) of the small parts shown by white arrows labeled (d) and (e) in Fig. 4.18c. The EDX spectrum (d) shows high Al and low Si concentrations, suggesting the existence of allophane and imogolite, whereas the EDX spectrum (e) is close to that of the non-colored volcanic glass (Fig. 4.15e). Thus, consistent with the brownish color of the pumice in Fig. 4.16b, partial weathering of the pumice particle is shown by the SEM-EDX analyses. In this example, weathering appears most intensive at the rim sites of the pumice particle.
4.4.3 A Horizon Soil with Andic Soil Properties
The micromorphological and chemical properties of the A3, 2Bw1, and 3Bw5 horizons were examined using thin sections and polished sections. The vegetation of the sampled site is Quercus Serrata and undergrowth (Otowa and Shoji 1987) (Fig. 4.19b). Thin sections of the A3 horizon (Fig. 4.19c) showed porous and granular microstructure (Stoops 2007) similar to those reported by Kawai (1969). In the 2Bw1 horizon (Fig. 4.19d), a coating of fine, light brown material is probably a more weathered part of pumice particles. Vesicular structure remains inside the pumice. The light gray parts of Fig. 4.19d are voids. In the 3Bw5 horizon (4.19e), small light brown and orange parts are found within the weathered pumice. These parts are similar in appearance to the orange-reddish altermorph reported for the Bw3 horizon of the Andosol of Tenerife by Stoops (2007). The light gray parts are also voids in Fig. 4.19e. The micromass of these three thin sections, Fig. 4.19c–e, showed undifferentiated b-fabric between crossed polarizers, indicating that isotropic or non-crystalline materials are dominant in the fine materials of these horizons.
Figure 4.21a shows a reticulated pattern of Fig. 4.20f with magnification, and the EDX spectrum of the dashed area is highly dominated by Al (Fig. 4.21b), whereas the EDX spectrum of the fine materials near the reticulated pattern suggests allophanic material (Fig. 4.21c). The Al element map of the reticulated area (Fig. 4.21d) shows that the Al concentration is nearly homogeneous with little Si present (Fig. 4.21e). These results suggest that the reticulated material is a sclerotia grain.
The formation of Al-humus, phytoliths, sclerotia grains, and diatoms results from biological activities in the A horizons. The formation of laminar opaline silica may be facilitated by the formation of Al-humus. Allophane, imogolite, and ferrihydrite are formed from inorganic parent materials not only in the A horizons but also in the Bw horizons of Andisols.
4.4.4 B Horizon Soil with Andic Soil Properties
In Fig. 4.26a, which is the magnified SEM image of dashed square Fig. 4.25d, vesicular structures remain inside the sponge-like volcanic glass . However, two types of vesicle walls can be distinguished: thick and thin. Two thin walls (indicated by red boxes labeled c and d in Fig. 4.26a) were found to be Al-rich according to their EDX spectra (Fig. 4.26c, d). These EDX spectra are close to that for allophanic material. In contrast, a thick wall (indicated by the blue box labelled b in Fig. 4.26a), has an EDX spectrum similar to that of non-colored volcanic glass like Fig. 4.1a–c. Examining the Al and Si element maps of Fig. 4.26a shown in Fig. 4.26e, f, respectively, thin walls in the Fig. 4.26a are Al-rich and Si-poor, whereas thick walls are Al-poor and Si-rich. These observations suggest that the thin walls of vesicles are allophanic altermorphs described by Stoops (2007) and Gerard et al. (2007). Comparing the element maps of Al and Si, Fig. 4.26e, f, respectively, it can be seen that allophanic altermorphs also surround the thick volcanic glass. Hence, the allophanic altermorph formed by releasing Si, Ca, Na, and other soluble elements from the surface of volcanic glass, and remains at the original site.
Figure 4.25e shows a phenocryst that displays cleavage. Examining the cleavage reveals (Fig. 4.27a) thin stripes along the edge of the remaining phenocryst. Examining EDX spectra of the thin stripe (dashed square (b) in Fig. 4.27a) and phenocryst (dashed square (c) in Fig. 4.27a) reveals that they are allophanic material and plagioclase, respectively. Examining Al (Fig. 4.27d) and Si (Fig. 4.27e) element maps, thin stripes of the plagioclase phenocrysts are evident. The Al concentration is higher than the Si concentration in these thin stripes as shown by the EDX spectrum (Fig. 4.27b). Two possibilities exist for the formation of the allophanic thin stripes. One is the formation of altermorphs of plagioclase and the other is precipitation of allophanic materials in the cleavage opening.
The magnified SEM image (Fig. 4.29a) shows that an altermorph of the vesicle walls remains and that materials with diffuse boundaries are filling part of the pores. The EDX spectra of the red squares (b) and (c) in Fig. 4.29a show that these are both Al-rich materials, possibly allophanic in nature. The Al (Fig. 4.29d) and Si (Fig. 4.29e) element maps show that no glassy parts remain. Although the peaks are small, the iron content is higher than that in Fig. 4.26c, d. The reddish yellow color of Fig. 4.28a and the 3Bw5 horizon of Fig. 4.19a is probably owing to iron or ferrihydrite in the allophanic aggregate. The oxalate-extractable Fe (Feo) content of the 3Bw5 horizon is 4.78%, and is even higher than the 2Bw1 horizon (Fig. 4.19a, 0.46%). The vesicle walls of the Fig. 4.29a are thin compared to the glass walls remaining in the Fig. 4.26a. Weathering is more intensive in Nt-I than in Nt-S even though the difference in age of the Nt-I and Nt-S is estimated to be small. Possible reasons are that (i) the glass wall was thin, and (ii) the iron content of the volcanic glass was high. Another difference between the 3Bw5 horizon and the scoria particles in Figs. 4.6 and 4.7 is the lack of phenocrysts in the former.
4.4.5 Changes in Elemental Composition with Andisol Formation
Two analyses were performed before constructing Fig. 4.30. One was a principal component analysis of element concentration data of 46 elements using 95 Andisol samples from 18 pedons with different rock types. The first principal component was suggested to be depletion and enrichment of elements, and the second principal component (PC2) was suggested to be related to rock type. Then, andesitic Andisols, for which the variation in PC2 scores was relatively small, was used for Fig. 4.30.
As Wp/Ws is calculated using Eq. (4.1), the red lines in Fig. 4.30 can be drawn so as to pass through the average concentration Ej,s and the average weight change Wp/Ws. If an element is immobile during Andisol formation, its concentration is plotted along this line. The theoretical Ej,p can also be calculated using Ej,s and Wp/Ws so long as an element is immobile. If a parent tephra has higher Ej,p than average, the plot of Ej,s will appear above the solid line, and if it has a lower Ej,p, it will appear below the solid line. If different Ej,p values of elements are scaled similarly, the slopes of their Ej,s plots will also be similar as seen in Fig. 4.30.
Among 54 elements, at least 27 (Be, Al, Ti, Fe, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Tl, Pb, Th, and U) were enriched in the Andisols, and the increase in these concentrations was related to total weight loss owing to the soil formation processes. Of the major elements in soil, concentrations of Si, Ca, and Na clearly decrease with the weight loss as shown in Fig. 4.30. However, the slope of the decrease in the element concentration is steeper for Ca and Na than for Si (Fig. 4.30). One possible reason is that Ca and Na are not the major constituents of Andisol formation products, but Si is the main structural constituent of allophane and imogolite. Other possible explanations are that Si can be sorbed by ferrihydrite, and that Si is also partly retained in Andisols as opaline silica or phytoliths. The weight-loss of tephra, maintaining the altermorphs (Figs. 4.26 and 4.29), may be the major reason for the reduction of bulk density in the Bw horizon of Andisols (Nanzyo et al. 2007).
4.4.6 Volcanic Ash Soils Under Various Drainage Conditions
The first step is to prepare a sample for both polarizing microscope and SEM observations. To do so, spread sample particles on a glass slide without overlapping. For the 0.05–0.02 and 0.02–0.005 mm fractions, place about 2–3 mg of air-dried powder on a glass slide by hand, and perform the following procedure: tilt the slide, holding one side of the slide, tap the other side of the slide on a table so that the particles gradually slide down and spread evenly on the slide.
Use one side of transparent double-sided sticky tape to pick up the particles from the glass slide, and stick the other side of the tape to a cover glass. A disk-shaped cover glass is more convenient for handling than square one. Put the cover glass on a glass slide, and then put the slide on the rotating stage of a polarizing microscope (Lynn et al. 2008). Take photographs with plain and crossed polarizers at least 2 different angles separated by 45° to distinguish isotropic (non-crystalline) and anisotropic (crystalline) particles (Fig. 4.12d). Use the photographed particles for the next step.
The second step is to observe the morphological properties of each particle using high-resolution SEM imaging (Fig. 4.12a). Coating with vacuum-evaporated carbon is desirable for obtaining EDX data. The third step is to obtain element maps (Na, Mg, Al, Si, P, K, Ca, Mn, Ti, and Fe) of the same SEM image. Free software packages are available for handling rotating polar microscope images, for overlaying element maps with other maps and photographs, and for counting particles. Among the isotropic particles, the particles having only Si and no other elements are plant opals. Among the isotropic particles, the particles having Al are volcanic glass. Examine the EDX spectrum of the particle in order to minimize identification errors.
- Dahlgren RA (1994) Quantification of allophane and imogolite. In: Amonette JE, Zelazny LW (eds) Quantitative methods in soil mineralogy. SSSA Miscellaneous Publication, SSSA, Madison, pp 430–451Google Scholar
- Drees LR, Wilding LP, Smeck NE, Senkayi AL (1989) Silica in soils: quartz and disordered silica polymorphs. In: Dixon JB, Weed SB (eds) Minerals in soil environments, 2nd edn. SSSA, Madison, pp 913–974Google Scholar
- Eden DN (1992) A standard method for determining volcanic glass content in Andisols. DSIR Land Resources Scientific Report No.2. DSIR Land Resources, Private Bag, Lower Hutt, New Zealand, pp 5–27Google Scholar
- Harsh J, Chorover J, Nizeyimana E (2002) Allophane and imogolite. In: Dixon JB, Schulze DG (eds) Soil mineralogy with environmental applications. SSSA Inc., Madison, pp 291–322Google Scholar
- Heiken G, Wohletz K (1985) Volcanic ash. University of California Press, Berkeley, pp 1–246Google Scholar
- Ishizaki Y, Morita T, Toriyama H (2017) Sequence and magma plumbing system of explosive eruptions that formed the Nantai-Imaichi tephra and the associated scoria flow deposits, Nantai volcano, NE Japan. Bull Volc Soc Jpn 62:95–116Google Scholar
- Kawai K (1969) Micromorphological studies of Andosols in Japan. Bull Ntnl Inst Agric Sci (Japan) 20:77–154Google Scholar
- Kondo R, Sase T, Kato Y (1988) Opal phytolith analysis of andisols with regard to interpretation of paleovegetation. In: Kinloch DI, Shoji S, Beinroth FH, Eswaran E (eds) Proceedings of the 9th international soil classification workshop, Japan. 20 July to 1 August, 1987. Publication by Japan Committee of 9th international soil classification workshop, for the soil management support services, Washington, DC, USA, pp 520–534Google Scholar
- Lynn W, Thomas JE, Moody LE (2008) Petrographic microscope techniques for identifying soil minerals in grain mounts. In: Ulery AL, Drees LR (eds) Methods of soil analysis. Part 5. Mineralogical methods, SSSA book series, vol 5. SSSA, Madison, pp 161–190Google Scholar
- Machida H (2002a) Quaternary volcanoes and widespread tephras. Glob Environ Res 6:3–17Google Scholar
- Machida H (2002b) Volcanoes and tephras in the Japan area. Glob Environ Res 6:19–28Google Scholar
- McDaniel PA, Lowe DJ, Arnalds O, Ping C-L (2012) Andisols. In: Huang PM, Li Y, Sumner ME (eds) Handbook of soil sciences, properties and processes, vol 33, 2nd edn. CRC Press, Taylor & Francis Group, Boca Raton-London-New York, pp 29–48Google Scholar
- Miyaji N (2002) The 1707 eruption of Fuji volcano and its tephra. Glob Environ Res 6:37–39Google Scholar
- Nanzyo M, Yamasaki S, Honna T (2002) Changes in content of trace and ultratrace elements with an increase in noncristalline materials in volcanic ash soils of Japan. Clay Sci 12:25–32Google Scholar
- Otowa M, Shoji S (1987) Ninth international soil classification workshop, properties, classification, and utilization of Andiosls and Paddy soils, Kanto, Tohoku, and Hokkaido, Japan, 20 July–1 August 1987, Tour Guide. Soil Management Support Survice, USDA Soil Concervation Service, Washington DC, Japanese Committee of the ninth international soil classification workshop, pp 241–251Google Scholar
- Parfitt RL, Wilson AD (1985) Estimation of allophane and halloysite in three sequences of volcanic soil, New Zealand. In: Caldas EF, Yaalon DH (eds) Volcanic soils, weathering and landscape relationships of soils on tephra and basalt, catena supplement 7. Catena Verlag, Braunschweig, pp 1–8Google Scholar
- Shoji S (1986) Mineralogical characteristics I. Primary minerals. In: Wada K (ed) Andosols in Japan. Kyushu University Press, Fukuoka, pp 21–40Google Scholar
- Shoji S, Nanzyo M, Dahlgren RA (1993) Volcanic ash soils – genesis, properties and utilization. Elsevier, AmsterdamGoogle Scholar
- Sievert L, Simkin T, Kimberly P (2010) Volcanoes of the world. University of California Press, Berkeley/Los Angeles, pp 48–49Google Scholar
- Sonehara T (2016) Chemical composition of volcanic glass samples determined by EPMA and XRF: examples of the Ito and Aso-4 pyroclastic flow deposits. Eng Geol J 6:5–19 (in Japanese)Google Scholar
- Wada K (1989) Allophane and imogolite. In: Dixon JB, Weed SB (eds) Minerals in soil environments, 2nd edn. SSSA, Madison, pp 1051–1087Google Scholar
- Wilding LP, Smeck NE, Drees LR (1977) Silica in soils: quartz, cristobalite, tridymite and opal. In: Dixon JB, Weed SB, Kittrick JA, Milford MH, White JL (eds) Minerals in soil environments. SSSA, Madison, pp 471–552Google Scholar
- Yamada I, Shoji S (1975) Relationship between particle-size and mineral composition of volcanic ashes. Tohoku J Agr Res 26:7–10Google Scholar
- Yoshinaga N (1986) Mineralogical characteristics II. Clay minerals. In: Wada K (ed) Andosols in Japan. Kyushu University Press, Fukuoka, pp 41–56Google Scholar
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this book are included in the book's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the book's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.